Lattice sites of implanted Cu and Ag in ZnO

The group $\textrm{I}$b impurities Cu and Ag on substitutional Zn sites are among possible candidates for p-type doping of ZnO. In order to explore possible lattice sites of Cu and Ag in ZnO the radioactive impurities $^{67}\!$Cu and $^{111}\!$Ag were implanted at doses of $4\!\times\!10^{12}$cm$^{-2}\to1\!\times\!10^{14}$cm$^{-2}$ at 60 keV into ZnO single crystals. The emission channeling effects of $\beta\!^{-}$ -particles from the decay were studied by means of position-sensitive electron detectors, giving direct evidence that in the as-implanted state large fractions of Cu and Ag atoms (60--70% for Cu and 30% for Ag) occupy almost ideal substitutional Zn sites with root mean square (rms) displacements of 0.014--0.017 nm. However, following vacuum annealing at 600 °C and above both Cu and Ag were found to be located increasingly on sites that are characterized by large rms displacements (0.03--0.05 nm) from Zn sites. We conclude that in high-temperature treated ZnO Cu and Ag are most likely not simply replacing Zn atoms but are incorporated in complexes with other crystal defects or as clusters.